Wireless Microphone Systems: Concepts of Operation and Design
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$15.95 Preface Wireless microphone systems are a key component in almost every broadcast, motion picture, theatrical and sound stage production, as well as corporate, religious and educational venues. As the airwaves continue to become more congested with a myriad of wireless devices and the demand for more wireless microphone systems increases, the need to understand the concepts behind the design and operation of wireless systems becomes a critical concern for professional users. The emergence of DTV (digital television) broadcasts in North America has created an increasingly difficult environment for operating wireless systems as the available spectrum becomes more congested. DTV is also emerging in Europe, as another indicator of the spectrum crowding that the future holds in store. Considering these facts, and the ever increasing popularity of wireless microphones, intercoms, IFB systems and other radio communications devices used in almost every production, the need for a solid, technical understanding of wireless systems is at an all-time high. This guide is intended to uncover some of the mysteries behind the operating principles of wireless systems and help the reader separate fact from fiction when selecting a wireless system for a particular application. Far too much propaganda has been published by various manufacturers, with boastful, and sometimes incredible claims about the quality of the equipment being offered. Armed with an understanding of the basic principles of wireless operation, it becomes possible to see through the maze and make intelligent choices. Our philosophy at Lectrosonics is to build the best product we know how to make, and support it with the best service possible. This includes the publication of guides like this one, and maintaining a highly responsive attitude toward the market. Please feel free to contact us regarding any of the material presented in this guide. Your comments, suggestions and experience are extremely valuable to us. Bruce C. Jones Vice-president, Marketing Lectrosonics, Inc. March 2000 Copyright © 2000 by Lectrosonics, Inc. Contents Page BASIC WIRELESS SYSTEM CONFIGURATION ............................................... 1 TRANSMITTERS.................................................................................................. 3 RECEIVERS ......................................................................................................... 7 DIVERSITY RECEPTION.................................................................................. 13 AUDIO SIGNAL PROCESSING ........................................................................ 19 ANTENNAS ........................................................................................................ 23 FREQUENCY SYNTHESIS ............................................................................... 27 INTERFERENCE ................................................................................................ 31 FREQUENCY COORDINATION........................................................................ 37 MULTI-CHANNEL WIRELESS SYSTEMS and MULTI-COUPLERS .............. 41 SIGNAL TO NOISE RATIO................................................................................ 45 INTERPRETING WIRELESS MIC SPECIFICATIONS................................... 47 EVALUATING WIRELESS MICROPHONE SYSTEMS .................................... 51 GLOSSARY OF WIRELESS MICROPHONE TERMS...................................... 55 WIRELESS MICROPHONE APPLICATIONS ................................................... 57 Basic Wireless System Configuration BASIC WIRELESS SYSTEM COMPONENTS A wireless microphone system is a highly specialized combination of RF (radio frequency) and audio electronics that replaces the cable normally used to connect a microphone to another audio component. There are basically three components in a wireless microphone system: Belt-pack 1) Microphone transmitter 2) Transmitter 3) Receiver The term “system” refers to a combination of these three components. The microphone in a wireless system may be an integral part of the transmitter Plug-on transmitter or it can be a separate component. In many cases, the microphone is a separate component purchased apart from the wireless system. A huge variety of differ- ent types of microphones are available to meet any application. The application normally dictates the type of receiver needed, and the microphone requirement normally dictates the transmitter model needed. Hand-held transmitter Wireless transmitters are available in three different configurations for various applications: • Belt-pack models • Hand-held transmitter/microphone combinations • Plug-on models Wireless receivers are also available in a variety of configurations for various applications: • Compact models for field production • Table-top models for sound reinforcement • Rack-mount models for studio and stage Compact receiver Table-top receiver Rack-mount receiver 1 Wireless Microphone Systems The total number of wireless systems needed in a single SPECTRUM USAGE AND FREQUENCIES location often points to the need for specialized RF distribu- The vast majority of high quality wireless microphone tion, antennas, cabling, receiver mounting assemblies and systems use a method of radio transmission called “FM” other accessories such as the following: (Frequency Modulation). In FM systems, the radio signal (the • Antenna multi-couplers carrier) is frequency modulated (the frequency is increased • Quad-pak receiver systems and decreased) by the audio coming from the microphone. Another method of radio transmission, “AM,” (Amplitude • Remote antennas and cabling Modulation) is commonly used for communications and • In-line RF filter/amplifiers and splitter/combiners voice-band applications. In general, FM produces better audio than AM, so the FM principle is used almost exclu- sively for wireless microphones. In the USA, wireless microphones operate in frequency bands Rack mount multi-coupler specified by the FCC (Federal Communications Commission). Bands have been allocated for wireless microphones in the VHF spectrum from 150 to 216 MHz, and in the UHF spectrum from 470 to 806 MHz. These bands are used almost exclusively for television broadcast, except for a small portion of the VHF band between 169 and 172 MHz. DTV (digital television) broadcasts are being allocated in the UHF spec- trum in what were formerly empty channels. The upper and Quad Pak multi-coupler lower parts of the UHF spectrum are also being broken up and re-allocated to make room for additional services. As the available spectrum space for wireless microphones continues to shrink, the need for higher quality wireless microphone systems increases dramatically. High powered television broadcast signals on adjacent channels can render a previously working wireless system virtually useless. High performance receivers and multi- couplers, and specialized antennas and cabling equipment are required for any professional application to meet the con- stantly increasing demand for more and more wireless microphone systems. As you can well imagine in this “digital” world, extensive engineering effort is also being applied to develop digital modulation techniques for wireless microphone applications. The hopes are that digital wireless systems might alleviate some of the problems encountered with the present analog FM systems that now exist, and produce equal or higher Folding dipole antenna quality audio performance. It is much too early (as of the date of this writing) to delve into the intricacies of compara- tive digital radio techniques, so this guide is focused on the FM principle. Present digital technology has produced good quality cellular telephone systems, but the limited audio bandwidth of even the highest quality telephone system cannot produce the audio quality needed for even minimal LPDA antenna wireless microphone applications. (Log Periodic Dipole Array) In-line RF filter/amplifier Passive 4-way RF splitter/combiner 2 Transmitters TRANSMITTERS OVERALL CONFIGURATIONS Transmitter designs can be grouped into three basic types: Plug-on “Belt-pack” lavalier models for use with lapel mics, instruments, mixers, tape decks, etc. “Plug-on” models for use with hand-held mics, boom poles, mixers and other equipment with XLR connectors. “Hand-held” models with an integral microphone capsule, primarily for hand-held use. Belt-pack “Plug-on” transmitters adapt any microphone with an XLR connector to wireless operation. Their usefulness is evidenced by the increasing number of models offered by various manufac- turers. Professional models typically operate on alkaline or lithium 9 Volt batteries, and provide a wide input adjustment range to accommodate the wide variety of microphone types. The mechanical construction, especially the input coupler, is the key to what makes this type of transmitter most useful for Belt-pack transmitters can be constructed of metal or professional applicaitons. plastic. They can be designed to utilize the micro- phone cord or input cable as the antenna (common in VHF designs), or use a “whip” antenna. Input gain must be adjustable over a wide range to accurately match the enormous variations in output levels from Hand-held microphones and other equipment. Most belt-pack transmitters operate on alkaline or lithium 9 Volt batteries, and include a battery status indicator of some type. The best designs offer adequate warning of low battery condition to allow time to plan for a battery change. The battery